How do I fill an area with faces lined up in a grid?
Delete all of the top faces.



Select the top loop edge ring.



Press CtrlF and select Grid Fill (or use the menu: Mesh->Faces->Grid Fill)



, playing with the settings until you find a result that you like:



NOTE:  For this to work you need a continuous loop with an Even number of vertices. An Odd number of vertices will report a "must select two edge loops" error.